1. Field of the Invention
The present invention relates to an overheat protection circuit that operates to suspend a circuit operation of a power supply integrated circuit in case of overheating.
2. Description of the Related Art
A power supply integrated circuit, typified by a series regulator or a switching regulator, contains an output transistor for allowing high current to flow. Accordingly, large power dissipation of the output transistor and insufficient heat dissipation of the integrated circuit involve a danger of smoke or fire due to overheating. For that reason, the power supply integrated circuit that handles high current is provided with a built-in overheat protection circuit for ensuring safety.
A widely-used example of the built-in overheat protection circuit for a power supply circuit is disclosed in Japanese Patent Application Laid-open No. 2005-100295 (FIG. 3).
A general overheat protection circuit employs a diode as a thermal element to utilize forward voltage temperature characteristics of the diode. In a case of using a parasitic diode to be formed through a CMOS process, a forward voltage of the diode is determined based on a bandgap voltage of silicon and has a temperature coefficient of approximately −2 mV/° C. independently of a process, and hence the diode is suitable for a thermal element on an integrated circuit.
Comparing an output of the thermal element with a reference voltage having no temperature coefficient enables detection as to whether the thermal element has exceeded a given temperature or not. The reference voltage is set to be equal to a voltage that is output from the thermal element at a temperature to be determined as overheat. The overheat protection circuit is configured to turn OFF an output transistor when overheat is detected based on the magnitude relation between the output voltage of the thermal element and the reference voltage.
FIG. 2 illustrates a circuit diagram of a power supply integrated circuit including a conventional overheat protection circuit. The power supply integrated circuit includes a voltage regulator 100 and an overheat protection circuit 101.
The overheat protection circuit 101 includes an enhancement/depletion (E/D) type reference voltage circuit 102, a reference voltage adjustment circuit 103, and a temperature detection circuit. The E/D type reference voltage circuit 102 outputs a reference voltage Vref0, which is input to the reference voltage adjustment circuit 103. The reference voltage Vref0 is input to an inverting input terminal of a comparator 21 as a reference voltage Vref via the reference voltage adjustment circuit 103. Input to a non-inverting input terminal of the comparator 21, on the other hand, is a forward voltage Vf of a diode 20 that is biased by a constant current source 23. The forward voltage Vf of the diode 20 biased with a constant current has a negative temperature coefficient of approximately −2 mV/° C. FIG. 3 illustrates respective relations of the voltages Vf and Vref with respect to a temperature Tj (junction temperature).
If the temperature Tj is low and Vf>Vref is satisfied, a detection signal VDET of the comparator 21 becomes High to turn OFF a P-type metal oxide semiconductor (PMOS) transistor 22. Accordingly, the voltage regulator 100 operates normally.
If the temperature Tj increases and Vf<Vref is satisfied, the output level of the comparator 21 becomes Low to turn ON the PMOS transistor 22. As a result, the voltage regulator 100 enters a shutdown state.
Through the adjustment to the reference voltage by means of the reference voltage adjustment circuit 103, the voltage regulator 100 may be shut down at a desired overheat detection temperature.
However, the overheat protection circuit configured as described above involves the following problems in improving temperature detection accuracy.
The reference voltage circuit leads to an increased area. In the case of employing an E/D type reference voltage circuit as a reference voltage circuit, there is a fluctuation in reference voltage of approximately 100 mV due to a fluctuation in threshold of MOS transistors. Therefore, trimming is required in a manufacturing process so that the reference voltage may be set to a desired voltage value. Consequently, additional reference voltage adjusting means for adjusting the reference voltage needs to be provided, resulting in an increased area. Even when a high-voltage precision bandgap reference is employed as a reference voltage circuit, a large number of diode elements and an error amplifier are required, resulting in an increased area.
Further, a random offset of the comparator 21 may be responsible for a fluctuation in detection temperature. In a case where the comparator 21 is formed through a MOS process, the comparator 21 has a random offset of approximately 10 mV.
When it is supposed that the comparator 21 has a random offset of ±12 mV and the temperature coefficient of the thermal element is −2 mV/° C., the fluctuation in detection temperature due to the random offset of the comparator 21 corresponds to ±6° C. In order to reduce the fluctuation in detection temperature due to the random offset of the comparator 21, it is conceivable to reduce the random offset of the comparator 21 or increase the temperature coefficient of the thermal element. Reducing the random offset of the comparator 21 involves increasing the size of transistors constituting the comparator 21, leading to an increased area. On the other hand, increasing the temperature coefficient of the thermal element causes a large fluctuation width of the output voltage of the thermal element in a range of from room temperature to high temperature at which overheat is detected, which is disadvantageous for low voltage operation.